The complement (C) system is an essential component of innate immunity and is actively involved in the host defense against infectious agents and in the removal of immune complexes and apoptotic cells (Walport, 2001, N Engl J Med, 344: 1140-1144). Tumor cells may also be potential target of C since C4, C3 and C5b-9 are deposited on breast and thyroid carcinoma. The C system has a definite potential advantage over cytotoxic cells as a defense system because it is made of soluble molecules that can easily reach the tumor site and diffuse inside the tumor mass. Moreover, C components are readily available as a first line of defense because they are synthesized locally by many cell types, including macrophages, fibroblasts and endothelial cells, and the amount of components released is regulated by cytokines and other pro-inflammatory molecules.
The C system requires an activation process to release the biologically active products that are capable of recognizing and attacking neoplastic cells. The system can be activated by tumor cells, as for example B lymphoblastoid cells that trigger the C sequence through the alternative pathway. Some glioma cell lines bind mannose binding lectin and activate the lectin pathway. Apoptotic tumor cell lines can also activate the alternative pathway by expressing a surface molecule undetectable on normal cells. However, antibody-mediated activation of the classical pathway represents the most efficient way to target C activation products to tumor cells in sufficient quantity to cause cell damage. Unfortunately, only low-titer and low-affinity antibodies (Abs) to tumor antigens are usually detected in cancer patients. These Abs are poor C activators and are therefore unlikely to mediate C-dependent cytotoxicity (CDC) of tumor cells.
A renewed interest in C as an important effector system for tumor cell cytotoxicity has been raised by the introduction of chimeric or humanized monoclonal Abs in cancer therapy. Several Abs have been developed and some of them are currently used in the treatment of patients with hematological malignancies and solid tumors (Gelderman, et al., 2004, Trends Immunol. 25: 158-164). Rituximab® is the most extensively studied chimeric Ab and has been used for the treatment of low-grade/follicular non-Hodgkin's lymphomas as a single therapeutic agent with a success rate of about 50%. This mouse/human chimeric Ab is directed against CD20 expressed on mature B lymphocytes and on 90% of B-cell non-Hodgkin's lymphomas and contains the C-fixing human IgG1 and κ regions.
The mechanisms implicated in the killing of lymphoma cell lines mediated by Rituximab include a direct apoptotic effect of the Abs, Abs-dependent Cell Cytotoxicity (ADCC) and CDC. However, these three mechanisms may not be equally effective in inducing tumor cell death in vivo. Analysis of primary non-Hodgkin's lymphoma cells for their susceptibility to Rituximab-mediated killing showed that Rituximab had a negligible apoptotic effect. Furthermore, the susceptibility of all the cells examined for ADCC did not correlate with the clinical response to the Abs, whereas the in vitro cell-sensitivity to CDC was found to be the best predictor of the in vivo effect of Rituximab. Additional support for the important role played by CDC in mediating the therapeutic effect of Rituximab was provided by Di Gaetano and colleagues (Di Gaetano et al., 2003, J Immunol. 171: 1581-1587) who showed that Rituximab inhibited the growth of a murine lymphoma transfected with CD20 in C sufficient mice, but not in C1q deficient mice.
A major limitation to the therapeutic efficacy of mAbs is represented by the surface overexpression of the C regulatory proteins (CRPs) CD46, CD55 and CD59 that inhibit the C sequence at different steps of activation. These CRPs restrict the susceptibility of tumor cells to Abs-dependent CDC and provide a mechanism of evasion for tumor cells to resist C attack. Golay and colleagues (Golay et al., 2001, Blood 98: 3383-3389) analyzed several B lymphoma cell lines and a few samples of fresh follicular non-Hodgkin's lymphoma cells for their sensitivity to CDC induced by Rituximab and observed that C resistance was dependent on the expression level of CD55 and CD59. The same group later extended these findings to freshly isolated B cells from patients with chronic lymphocytic leukemia. In both of these studies lyses of the C-resistant cells was restored by the addition of neutralizing antibodies to CD55 and CD59 suggesting that blocking the inhibitory activity of the two CRPs would enhance the therapeutic effect of Rituximab.
Patent application US2003219434 discloses a method for making antibodies, for example antibodies directed against decay accelerating factor (DAF) (CD55), using a naive antibody phage library. The human antibody LU30 is suggested for use in assessing overexpression of DAF and for treatment of lung cancer particularly when combined with cytotoxic agents. Patent application WO2004048413 discloses the use of an antibody which binds to both complementary determining region SCR1 and SCR2 of CD55 in the treatment of tumors and leukaemia. Patent application WO9732021 describes the production of the anti-idiotype antibody 105AD7 and potential therapeutic uses of the antibody.
Therapeutic studies with antibodies directed to complementary determining region (SCRs) have been limited to immunoconjugated molecules (EP0552142, and Byers et al., 1987, Cancer Res 47: 5042-5046). Byers et al., describes studies with 791T/36 linked to ricin A chain and showed significantly inhibition of tumor growth in athymic mice. 791T/36-RTA was therefore screened in a phase I clinical trial in advanced colorectal cancer patients (Byers et al., 1989, Cancer Research 49: 6153-6160). However the trial was unsuccessful due to dose limiting toxicity.
Despite the evident progress, there remains a continued need for improved molecules able to elicit an immune response to human cancer cells in general and to cancer cells positive for CD55 and CD59 overexpression in particular.